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					       Chapter 1


Telecommunication means communication at a distance.
Data refers to information presented in whatever form.
Data communications exchange of data between two
devices via some form of transmission medium such as a
wire cable.


 Data Representation
 Data Flow

                      Data communications
 • Communicating devices must be part of a
   communication system made up of a combination
   of hardware (physical equipment) and software
 • Characteristics of data communications system:
      –   Delivery (Deliver data to correct destination)
      –   Accuracy (Deliver data accurately)
      –   Timeliness (Deliver data in a timely manner)
      –   Jitter (variation in packet arrival time, uneven delay in
          the delivery of audio or video packets.)

          Components of data communications
 • Message:
      – The information (data) to be communicated. Popular forms of information
        include text, numbers, pictures, audio, and video.
 • Sender:
      – The device that sends the data message.
      – It can be a computer, workstation, telephone handset, video camera, and so
 • Receiver:
      – The device that receives the data message.
 • Transmission medium:
      – The physical path by which a message travels from sender to receiver.
      – e.g., twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
 • Protocol:
      – A set of rules that govern data communications.
      – It represents an agreement between the communication devices.

      Five components of data communication

                        Data representation
 • Text
      – Text is represented as a bit pattern, a sequence of bits (0s or 1s).
      – The process of representing symbols is called coding.
 • Numbers-represented by bit patterns.
           - number is directly converted binary number.
 • Images
      – RGB (red, green, blue)
      – YCM (yellow, cyan, and magenta)
 • Audio
   - recording or broadcasting of sound or music.
 • Video
   -recording or broadcasting of a picture or movie.

                           Data flow
 • Communication between two devices can be
   simplex, half-duplex, or full-duplex (as shown in
   next slide).
 • Simplex
      – The communication is unidirectional, as on a one-way
 • Half-duplex
      – Each station can both transmit and receive, but not at
        the same time.
 • Full-duplex
      – Both stations can transmit and receive simultaneously.

  Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)


A network is a set of devices (nodes) connected by
communication links.
A node can be a computer, printer, or any other device
capable of sending and/or receiving data generated by
other nodes on the network.

                       Distributed processing
 • Distributed processing, - task is divided among
   multiple computers.
 • Network Criteria
       – Performance(transit time, response time)
          • Throughput
          • Delay
       – Reliability
          • Measured by the frequency of failure
          • The time it takes a link to recover from a failure
          • The network robustness in a catastrophe
       – Security

                   Types of connections
 • A network is two or more devices connected through links.
 • Link: Physical communication pathway that transfers data from
   one device to another.

 Two types of connections:
               1. Point-to-Point
               2. Multipoint

1) Point-to-point:
-dedicated link between two devices.
-entire capacity of the channel is reserved for transmission.
Eg : Changing TV channels by infrared remote control.

2) Multipoint: (Multidrop)
-more than two specific devices share a single link.
-Channel capacity is shared (either spatially or temporally)
-devices uses link simultaneously-spatial
-devices take turns-temporal

       Types of connections

                     Physical Topology
 • Physical topology: in which a network is laid out physically.
 • Two or more devices connect to a link; two or more links
   form a topology.
 • The topology of a network is the geometric representation
   of the relationship of all the links and linking devices
   (usually called nodes) to one another.
 • Four basic topologies:
              »   Mesh
              »   Star
              »   Bus
              »   Ring

       Categories of topology

                   Mesh Topology
 • Dedicated point-to-point link.
 • ie. Link carries traffic only between two devices it
 • MN:- n(n-1)/2 physical channels to link n devices.
        -every device must have n-1 i/o ports.

       A fully connected mesh topology (five devices)

                                 Mesh Topology

 • advantages
       – dedicated links (guarantees each link carries own data)
       – Robust (if one link is damaged, others work)
       – privacy or security (only intended recipient sees)
       – Ease fault identification and fault isolation
 •     disadvantages
       – The amount of cabling and the number of I/O ports required
       – Because every device connected with every other device, installation and
          reconnection are difficult.
       – wiring can be greater than the available space (in walls, ceilings, or floors)
       – h/w required to connect each link (I/O ports and cable) is expensive.

       Pbm: LD corporation consists of eight devices. Calculate total no. of cable
         links and no. of ports for each devices.

                       Star Topology
 • Each device has a dedicated point-to-point link only to a
   central controller, usually called a hub.
 • if one device wants to send data to another, it sends the
   data to the controller, which then relays data to the other
   connected device.

                                 A star topology connecting four stations

   •A star topology is less expensive than a mesh topology.
   •Easy to install and reconfigure
   •Far less cabling needs to be housed
   •Robustness: If one link fails, only that link is affected.

   •The dependency of the whole topology on one single point, the hub.
   If the hub goes down, the whole system is dead.

                       Bus Topology
 • A bus topology, on the other hand, is multipoint.
 • One long cable acts as a backbone to link all the devices in
   a network.
 • Nodes are connected to the bus cable by drop lines & taps.
 Dropline: Connection running b/w device & main cable.
 Tap : Connector either splices into the main cable.

                A bus topology connecting three stations

    •Ease of installation
    •Less cabling
    •Redundancy is eliminated
    •Difficult reconnection
    •fault isolation
    •Optimally efficient at installation, difficult to add new devices
    •Signal reflection at the taps can cause degradation in quality
    [A fault or break in the bus cable stops all transmission. The
    damaged area reflects signals back in the direction of origin,
    creating noise in both directions.]

                             Ring Topology
 • Each device is linked to only its immediate neighbors
   (either physically or logically).
 • Each device in ring has a repeater.
 • Advantages
   - Relatively easy to install and reconfigure
       – To add or delete a device requires changing only two connections.
       – Fault isolation is simplified.
       – A signal is circulating at all times. If one device does not receive a
         signal within a specified period, it can issue an alarm.
 • Disadvantages
       – Unidirectional traffic
       – In a simple ring, a break in the ring (such as a disable station) can
         disable the entire network.
       – It can be solved by using a dual ring or a switch capable of closing
         off the break.

              A ring topology connecting six stations

 • To connect n devices, n cable links are needed.
 • Eight device ring needs eight cable links.

  A hybrid topology: a star backbone with three bus networks

 • An entity is anything capable of sending or
   receiving information.
 • A protocol defines what is communicated, how it is
   communicated, and when it is communicated.
 • Syntax
       – Refers to the structure or format of the data, meaning
         the order in which they are presented.
 • Semantics
       – Refers to the meaning of each section of bits.
 • Timing
       – When data should be sent and how fast they can be sent.


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